The ability to control the speed of chemical reactions and in particular to increase the rate of chemical reactions is a matter of intense industrial interest. Rate of reaction is typically a function of temperature in which the higher the temperature the faster the reaction rate. In certain systems raising reaction temperature can have adverse effects. Other methods of changing the rate of reaction are desirable.
In equilibrium reaction systems two reactions are active simultaneously. A forward reaction produces a desired product and usually a less desired byproduct from original reactants. In addition the product and byproduct compounds can react together in a reverse reaction to re-form the original reactants. In such systems it is usually desired to increase the rate of production of the desired product. Increasing the temperature can increase the rate of the reverse reaction as well as the forward reaction. Consequently raising temperature does not necessarily convert the original reactants to the desired product faster.
Removing byproduct from the reaction mass of an equilibrium reaction can drive the reaction to high conversion by robbing the reaction mass of the product and byproduct materials that are the reactants for the reverse reaction. For equilibrium reactions in which water or methanol is a byproduct, removing water or methanol from the reaction mass during reaction can drive the reaction to more completely consume the original reactants to great productive benefit. Esterification, acetalization and ketalization condensations are examples of reactions that are normally limited in ability to provide purer product at higher yield and with greater speed due to equilibrium considerations. Water byproduct present in the reaction mass shifts the equilibrium unfavorably. However, if water could be removed, the equilibrium would shift further toward the product side of the reaction equations. There is a need to remove water from these reaction compositions at high rate to promote productivity of equilibrium reactions.
Traditional methods of removing water or methanol from other components include fractional distillation, thermal evaporation, cryogenic dehydration, and chemical adsorption to name a few. Such methods have drawbacks such as requiring generally complex equipment and systems (e.g., distillation columns with associated pumps, heat exchangers and the like). They typically involve recirculating fluid and solvents in large volume relative to the product volume which adds to material cost as well as contributes to potential waste, safety and environmental protection concerns. Also these processes call for substantial energy input for heating and cooling of circulating fluids that further adds to the cost of operation. Chemical adsorption processes frequently operate cyclically and therefore additionally often utilize oversized and redundant adsorber units so that saturated units can be taken off-line for regeneration without interrupting production.
Membrane separation processes for segregating components of mixtures by selectively permeating individual components through a membrane are well known. An excellent recent survey of membrane pervaporation and vapor permeation processes is presented in “Pervaporation Comes of Age” N. Wynn, Chemical Engineering Progress, pp. 66-72, October, 2001. Very basically, in such processes one side of a selectively permeable membrane is contacted with the fluid mixture of components to be separated. A driving force, such as a pressure gradient across the membrane in the case of vapor permeation and concentration gradient in the case of pervaporation, causes preferentially permeating components to migrate through the membrane such that a permeate composition enriched in the faster permeating components develops on the other side of the membrane. A retentate composition on the feed mixture side of the membrane becomes depleted in the faster permeating components. With a vapor feed mixture fluid, this process is generally known as vapor permeation. When the feed fluid is in the liquid state, low pressure, usually vacuum, vaporizes the migrating components on the permeate side. This technique is known as pervaporation.
While offering a valuable alternative to other water and methanol removal methods, existing membrane separation technology also has limitations. Productivity is constrained by the separation characteristics of the membrane. It is a long standing problem in this field that permselective membranes usually have either high transmembrane flux or high selectivity but rarely both. The term “selectivity” means the ratio of permeability through a membrane of a faster permeating species divided by the permability through the membrane of a slower permeating species. Thus the artisan must often choose a membrane material that sacrifices permeate flow rate to achieve an acceptable selectivity.
Chemical reaction productivity is another field in which removal via membrane of water or methanol has yet to be applied to fuller potential as described in the article by Wynn mentioned above. The article points to esterification, acetalization and ketalization condensations as examples of reactions that are normally limited in ability to provide purer product at higher yield and with greater speed due to equilibrium considerations. Conventionally hydrophilic composition membranes have been found to provide improved conversion in equilibrium reaction systems. Further improvements in equilibrium reaction effectiveness is desirable.